In the rubber tire calendering art, control of the manufactured sheet of tire material within precise limits is of highly significant concern. The production of rubber-coated cord within close tolerances is critical both from the standpoint of the quality of the manufactured article - optimum tire appearance, controllability, safety, and longevity - and from the standpoint of reducing the costs associated with production - minimized use of raw materials, minimized production of off-spec material, and minimized unproduction machine and manpower time. A further significant result of more accurate production control is the potential for a higher production rate brought about by a heightened confidence in the ability of the system to meet target specifications.
In the usual tire calendering process, a tire cord, comprising a layer of textile, metallic, or synthetic material, is forcibly laminated between two layers of rubber by means of opposing calender rolls. These layers are usually of equal thickness, though, of course, this need not be the rule. The positioning of the tire cord within the rubber tire material laminate must be precisely controlled within close tolerances, for the reasons given above relating to tire quality and production economics. Assurance that such positioning is in fact attained is had by maintaining a relatively constant thickness in the rubber layers above and below the tire cord. In addition to the exact positioning of the cord within the rubber layers, it is also highly important that the total thickness of the calendered tire material be maintained as close as possible to a selected total thickness target. This is necessary both from the point of view of the tire building process, for which the tire material is preferably at a constant total thickness, as well as the ultimate quality of the finished tire, in that a tire having insufficient rubber will not wear properly whereas one having an excess of rubber will be subject to excessive heat build-up from internal friction forces.
In referring to the amount of rubber deposited on either side of the tire cord, the terms "thickness" and "weight" are used interchangeably in the rubber art, since if the density of the rubber material is known, its thickness can readily be calculated from a measurement of its weight per unit area. To simplify the presentation which follows, the term "thickness" will be used exclusively, with the express reservation that alternative forms of measurement are not thereby relinquished.
In a sense, this application is directed to a refinement of the invention set forth in the copending application entitled "Process Control" of R. Heiks et al. The herein-disclosed invention, insofar as a preferred embodiment is described, comprises a single point gauge situated at the calender ahead of the final laminating rolls for measuring the thickness of the bottom rubber layer, and a scanning gauge located downstream from the final laminating rolls for measuring the final thickness of the composite rubber tire material. The respective measurements from the single point and scanning gauges are closely coordinated, with the result that they represent a sampling of bottom thickness and total thickness, respectively, taken from substantially the same longitudinal region of the material strip. Coordinated measurements over the same strip region are attained by programming the single point gauge to measure for an amount of time substantially equal to the time of one scan of the scanning gauge.
A bottom layer control signal, which may be derived from an indication of bottom thickness different from that which is derived from the coordinated measuring sequence, is used as the basis of control actions to maintain bottom layer thickness substantially on target. A top layer control signal, which is derived from the coordinated measurements of bottom and total thickness, is used to control the top layer thickness in such a manner as to maintain the total thickness of the strip substantially on target.
In the material coating art, it is known to position a scanning gauge on either side of a material strip coating element, whereby a thickness measurement of the material strip before coating is subtracted from a total thickness measurement of the identical portion of the strip after coating. A delay means connected to a means for measuring sheet velocity may be utilized to delay the scanning action of the scanning gauge situated downstream from the coating element with respect to the scanning action of the upstream scanning gauge, so that portions of the material strip are measured before and after coating. U.S. Pat. No. 3,190,261 issued June 22, 1965, to G. F. Ziffer, and U.S. Pat. No. 3,378,676, issued Apr. 16, 1968, to J.C. Clement, assigned to the same assignee as the disclose appliciation, discloses different forms of such a material coating control system.
However, the problem in the tire calendering industry of measuring and controlling the thickness of an upper layer of rubber overlying a tire cord and a lower layer of rubber is a more difficult one.
The lower rubber layer in a tire calendering system is produced in the process immediately prior to its joinder with the upper layer, and is susceptible to rather frequent variations in its thickness, because of changing conditions such as temperature, composition of the rubber stock, etc. This, in contrast to the situation in a material coating system where a base of relatively constant dimensions is involved, may require the capability for a higher frequency of control action over the lower layer than that ordinarily attainable through the typically slower measuring period of a scanning gauge system.
Other considerations, as well, may make the use of a scanning gauge to measure bottom thickness impractical for certain calendering systems. These involve the lack of space around the calender rolls, as well as factors affecting gauge response such as irregularities along the roll surface, problems of gauge standardization, and difficulties in gauge mounting and alignment.
The tire fabric is produced by combining the two rubber layers and the cord. Due to the action of the calender on these materials, the control to a desired strip thickness requires a unique control system that relates the layer control to the total and bottom thickness measurements and controls bottom thickness.
The present invention offers a unique and precise solution to the tire calendering problem of maintaining a desired bottom thickness, total thickness, and tire cord position relative to the top and bottom layers of rubber. Additionally, it resolves the problems, peculiar to a rubber tire calendering system, involved in coordinating the bottom and total thickness measurements over the same longitudinal region of the material strip, to obtain an accurate indication of the amount by which the thickness of the top rubber layer must be altered to maintain the total thickness of the material strip at the desired total thickness target, as will be apparent from a reading of the description which follows.